Most propane conversions get much better fuel economy than the 72-73% expected by comparing energy contents. With carbureted systems, I think it is not uncommon for an open-loop propane conversion to get 75-80% of the vehicle's gasoline economy. With feedback carburetor systems, I believe the typical propane fuel economy is about 80-85% of the vehicle's gasoline economy. Injection conversions on OBDII vehicles do even better with expected fuel economies in the 85-95% range.
With the open-loop carburetion systems, my understanding that the better than expected fuel economy is mainly due to improved, leaner fuel mixture distribution and a more ideal ignition advance curve. A gasoline carburetor system can have a significant variation in fuel mixture from cylinder to cylinder so the average fuel mixture must be richer overall to ensure that each cylinder gets a combustible fuel mixture. Closed loop carburetor systems improve on this mainly by ensuring that the fuel mixture remains constant over a wide range of engine temperatures.
Injection conversions on OBDII vehicles can't improve on fuel mixture distribution because of gasoline port injection. Properly configured, the fuel trims on CNG should the same as on gasoline. Because this type of conversion works by intercepting the gasoline injector signals, modifying them, and then rerouting them to the CNG injectors, theoretically this should result in CNG fuel economy that is closer to 100% of the gasoline economy. However, they could potentially do better because the vehicle's PCM should allow more ignition advance due to CNG's higher octane number. The problem is that the increased ignition advance may not necessarily be ideal for CNG.
CNG conversions tend to have noticeably less power than what the vehicle had on gasoline or even on propane because natural gas reduces the volumetric efficiency of an engine by displacing air. However, since the energy content of CNG is advertised in terms of GGE/GLE and since CNG is a high-octane gaseous fuel like LPG, my expectation is that the vehicle should similarly have an equivalent fuel economy better than what it would have on gasoline.
- Does anyone have any other explanations as to why CNG fuel economy should be better or worse than expected?
- What fuel economy are you seeing on CNG and what would you expect it to be on gasoline?